1. Field of the Invention
The present invention relates to a liquid crystal display device, and more particularly to a technique for setting a proper distance between a TFT substrate and a counter substrate by spacers.
2. Background Art
In a liquid crystal display device, liquid crystal is filled in a space formed between a TFT substrate on which pixel electrodes and thin film transistors (TFT) are formed and a counter substrate on which color filters and the like are formed, and images are formed by controlling molecules of the liquid crystal by an electric field which is generated between a pixel electrode and a counter electrode in response to a video signal. The distance between the TFT substrate and the counter substrate is extremely narrow, that is, several microns.
Conventionally, a distance between a TFT substrate and a counter substrate has been maintained by dispersing small beads or the like in the space formed between the substrates. However, a method which controls the distance by dispersing the beads in the space forms places where the number of beads is large and places where the number of beads is small. As a result, the distance between the TFT substrate and the counter substrate becomes non-uniform. When the distance between the TFT substrate and the counter substrate becomes non-uniform, there arises a drawback that a contrast of an image displayed on the liquid crystal display device is lowered or a drawback that an image becomes irregular.
To cope with such a non-uniform distance, as a method of defining a distance between a TFT substrate and a counter substrate, there has been developed a method (a support column method) which mounts columnar spacers formed of an organic film on the counter substrate. Since the columnar spacers are fixed to the counter substrate, it is possible to control the distance between the TFT substrate and the counter substrate in a stable manner. Further, in the columnar spacer method, when liquid crystal is filled into a space by a method which fills liquid crystal in the space by dropping the liquid crystal (liquid-crystal dropping filling method), differently from the method which uses the beads, the positions of the columnar spacers are not displaced at the time of dropping liquid crystal. Accordingly, the columnar spacer method is preferable.
Multi-layered lines which are provided for forming TFTs, video signal lines, scanning lines and the like are formed over a TFT substrate, and contact holes for connecting multi-layered lines with each other are formed in the TFT substrate. Portions of the TFT substrate where the contact holes are formed constitute recessed portions. On the other hand, columnar spacers are formed over the counter substrate. When the columnar spacers enter the contact hole portions, it is difficult to maintain a predetermined distance between the TFT substrate and the counter substrate. JP-A-2005-345819 (patent document 1) discloses the constitution where although it is desirable that the columnar spacers do not enter the contact holes, even when some columnar spacers may enter the contact holes, remaining columnar spacers which do not enter the contact holes maintain the distance between the TFT substrate and the counter substrate.
Besides the contact hole portions, convex portions are also formed over portions of the TFT substrate such as portions where lines intersect with each other and hence, a surface of the TFT substrate is not flat. JP-A-2005-242310 (patent document 2) discloses the constitution where columnar spacers having a fixed height are formed over a counter substrate, wherein some columnar spacers are brought into contact with a flat portion of the TFT substrate, and other columnar spacers are brought into contact with the convex portions. That is, patent document 2 discloses the constitution where other columnar spacers are compressed by an amount corresponding to the convex portions formed of lines or the like, and this compression amount acts as a so-called gravity margin. In this case, a compression amount of other columnar spacers is approximately 200 nm to 600 nm.